Multi-rotor electric machine

ABSTRACT

A multi-rotor electric machine having a stator, a first rotor magnetically coupled to the stator and a second rotor magnetically coupled to the stator is disclosed. A method of operating the electric machine comprises driving a gear via a first face of the gear using the first rotor and driving the gear via a second face of the gear using the second rotor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication No. 62/723,515 filed on Aug. 28, 2018, the entire contentsof which are hereby incorporated by reference.

FIELD

This relates generally to electric machines, and more particularly tomultiple-rotor electric machines such as motors and generators.

BACKGROUND

Electric machines with multiple rotors are known and may provideenhanced power over conventional electric machines. However,multiple-rotor electric machines may be heavier and less powerful thannecessary or desirable, and may not be as efficient as desirable.

Accordingly, there is a need for lighter and more powerful electricmachines which make more efficient use of materials.

SUMMARY

According to an aspect, there is provided a multi-rotor electric machinecomprising: a stator; a first rotor magnetically coupled to the statorand rotatably mounted relative to the stator; a second rotormagnetically coupled to the stator and rotatably mounted relative to thestator; and a gear drivingly coupled to both the first and secondrotors, the first rotor being drivingly coupled to a first face of thegear and the second rotor being drivingly coupled to a second face ofthe gear.

According to another aspect, there is provided an electric machinecomprising: a plurality of stators circumferentially distributed about acentral axis of the electric machine; a plurality of rotors arranged intriplets, each triplet of rotors sharing a common magnetic circuit witha respective one of the stators, each triplet of rotors comprising anadjacent pair of radially-outer rotors and a radially-inner rotorrelative to the central axis; and a common gear rotatable about thecentral axis and drivingly coupled to the plurality of magnetic rotors,the radially-outer rotors being drivingly coupled to a radially-outerface of the common gear and the radially-inner rotors being drivinglycoupled to a radially-inner face of the common gear.

According to another aspect, there is provided a method of operating anelectric machine having a stator, a first rotor magnetically coupled tothe stator and a second rotor magnetically coupled to the stator, themethod comprising: driving a gear via a first face of the gear using thefirst rotor; and while the gear is driven using the first rotor, drivingthe gear via a second face of the gear using the second rotor.

Other features will become apparent from the drawings in conjunctionwith the following description.

BRIEF DESCRIPTION OF DRAWINGS

In the figures which illustrate example embodiments,

FIG. 1 is a schematic perspective view of portions of an embodiment of amulti-rotor electric machine;

FIG. 2 is a schematic front cut-away view of portions of the electricmachine of FIG. 1;

FIG. 3 is a schematic side view of a cross-section of a portion of anembodiment of a multi-rotor electric machine;

FIG. 4 is a schematic front partial cut-away view of a portion of theelectric machine of FIG. 1;

FIG. 5 is a schematic front partial cut-away view of the portion of theelectric machine of FIG. 4 showing representations of a magnetic circuitassociated with the electric machine;

FIG. 6 is a schematic front partial cut-away view of a portion of anelectric machine showing rotor orientations in accordance with anembodiment; and

FIGS. 7A to 7E are schematic front partial cut-away views of the portionof the electric machine depicted in FIG. 6 throughout a half-cycle ofoperation.

DETAILED DESCRIPTION

Various aspects of preferred embodiments of electric machines accordingto the disclosure are described herein with reference to the drawings.

Electric machines may have more than one rotor. An example of amulti-rotor electric machine is provided in U.S. Pat. No. 8,232,700 B2,the contents of which are incorporated by reference in their entirety.

FIG. 1 is a schematic perspective view of an embodiment of an electricmachine. As depicted, electric machine 100 includes central shaft 104connected to common main gear 150. For example, central shaft 104 may bedrivingly coupled to main gear 150 via one or more radially-extendingarms or a circumferentially-continuous surface. FIG. 1 shows a singleradially-extending arm for clarity to provide visibility to internalcomponents of electric machine 100.

Main gear 150 engages with multiple (e.g., pinion) gears 118, 119arranged so as to be in meshing engagement with teeth on an outer faceof main (e.g., ring) gear 150 and an inner face of main gear 150. Eachgear 118, 119 is connected to a respective rotors 102, 103 via a rotorshaft 116. One or more windings 108 (see FIG. 2) are wrapped aroundstator 122 to induce a magnetic field (when current is applied towinding(s) 108) or to have a magnetic field induced therein (when maingear 150 rotates, thereby causing rotors 102, 103 and consequently gears118, 119 to rotate).

FIG. 2 is a schematic front view of the example electric machinedepicted in FIG. 1. As illustrated, machine 100 comprises a plurality ofouter magnetic rotors 102, a plurality of inner magnetic rotors 103,windings 108, stators 122, main gear 150, and shaft 104. As depicted inFIGS. 2 and 3, machine 100 includes a plurality of outer magnetic rotors102, each configured to rotate with a separate rotor shaft 116, and aplurality of inner magnetic rotors 103, each configured to rotate with aseparate rotor shaft 116. In some embodiments, the number of innerrotors 103 may be half the number of outer rotors 102 in machine 100.

As depicted in FIGS. 2 and 3, each rotor shaft 116 is configured todrive shaft 104 by means of outer (e.g., pinion) gears 118 and inner(e.g., pinion) gears 119 interacting with central main (e.g., ring) gear150. The rotor shafts 116 are caused to rotate by respective magneticrotors 102, 103. In some embodiments, the machine 100 is operable as amotor and current is applied to windings 108 to cause the gears 118, 119to drive the main gear 150. In some embodiments, the machine 100 isoperable as a generator so that when a torque is applied to shaft 104,main gear 150 causes rotors 102, 103 to rotate via gears 118, 119 andthus causes a flow of electrical current in windings 108.

Please note that in some figures, gears are shown without teeth for thesake of clarity. As described herein, gears may be provided in anysuitable form, including in the form of toothless wheels engaged byfriction, as well as gears with teeth which engage with other gears.

FIG. 3 is a schematic side view of a cross-section of a portion of anembodiment of an electric machine 100. As depicted, each rotor shaft 116is rotatably supported by front plate 134 and back plate 136, withsuitable bearings. Rotor shafts 116 may be formed integrally with orotherwise be connected to or coupled to a gear (e.g. outer gear 118 orinner gear 119). Outer gears 118 are configured to engage an outer faceof main gear 150. Inner gear 119 is configured to engage an inner faceof main gear 150.

Main gear 150 is connected to shaft 104, such that rotation of one ormore outer rotors 102 and/or inner rotor 103 causes outer gears 118and/or inner gear 119 to drive main gear 150, and therefore shaft 104,into rotation—or vice versa, depending upon the mode of operation (motorvs. generator). During a motor mode of operation, outer rotors 102 andinner rotors 103 can be used simultaneously to drive main gear 150.Alternatively, during a generator mode of operation, rotation of maingear 150 can drive outer rotors 102 and inner rotors 103 when generatingelectricity in windings 108.

In some embodiments, outer rotors 102 and inner rotor 103 are configuredto operate in electromagnetically independent triplets 160. That is, therotors 102, 103 can be separated magnetically into triplets 160, suchthat there is no provision of magnetic material linking any two triplets160 of rotors 102, 103 together, and the only linkages between separatetriplets 160 are mechanical (e.g. support structure, gears 118, 119 orother mechanical couplings).

In some embodiments, each triplet 160 includes two radially-outer rotors102 and one radially-inner rotor 103. The outer rotors 102 and innerrotors 103 of a given triplet 160 can benefit from the provision ofcommon magnetic circuit components, such as stators 122 and/or windings108, as shown, for example, in FIGS. 4 and 5. Such a sharedconfiguration can significantly reduce the amount of magnetic materialrequired for operation of the rotors, with corresponding cost and weightsavings. Such arrangement can also promote an efficient use of space andpower-density. Relative to other multi-rotor electric machineconfigurations, the use of both outer and inner rotors 102, 103 withshared magnetic components to engage main gear 150 on both inner andouter faces may increase the power output by as much as 50% withoutsignificant addition of weight to machine 100. Thus, the power-to-weightratio of machine 100 may be substantially increased relative to electricmachines which do not incorporate the disclosed configuration of outerrotors 102 combined with an inner rotor 103.

For example, since the magnetic circuit for the two outer rotors 102 andinner rotor 103 in triplet 160 is provided in common (see, e.g., FIG. 5which illustrates the magnetic flux path within the magnetic circuit fora given triplet 160 of rotors), the source of magnetic energy (winding108) may also be common to the three rotors in the triplet 160, and assuch shared by the three rotors 102 a, 102 b, 103 in triplet 160. Thismeans that the three rotors 102 a, 102 b, 103 in a triplet 160 can beenergized by a single winding 108, if desired, which results in asubstantial weight savings in the weight of the overall machine.Although the figures depict a configuration in which there is onewinding 108 per stator 122, it is contemplated that other embodimentsmay include more than one winding 108 per stator 122 for each triplet160 of rotors 102, 103.

Referring to FIG. 4, each rotor 102 a, 102 b, 103 comprises one or moremagnets 128 mounted on a shaft 116 and retained, particularly whenrotating, by a containment sheath 126. Magnets 128 comprise north andsouth poles (denoted “N” and “S”, respectively). In some embodiments,rotors 102, 103 comprise single pairs of north and south poles, and maybe referred to as bi-pole rotors 102, 103. Moreover, rotors are providedin triplets 160, each triplet 160 comprising a pair of outer rotors 102(denoted as first outer rotor 102 a and second outer rotor 102 b) and aninner rotor 103. The rotors are indexed such that magnets 128 aremounted, and rotate, (a) as individual rotors in a desired phase withrespect to their triplet-mates (102 a, 102 b, 103), and (b) by triplet160, in a desired triplet phase with respect to other triplets 160 a andwindings 108 a.

When using triplet 160 sets of bi-pole rotors indexed as describedherein, particular advantage may be gained by phasing rotors within eachtriplet 160. Such a configuration may make efficient use of the fluxpaths 132 (denoted in FIG. 5) around the rotors 102, 103 and thereforeprovide better efficiency of interactions between rotors 102, 103 andwinding 108, resulting in greater power being developed from electricmachine 100.

It should be noted that during operation, the direction of rotation ofinner rotor 103 is opposite to that of the outer rotors 102 a, 102 b.This is illustrated In FIG. 4, which shows the first and second outerrotors 102 a, 102 b rotating in direction A, while inner rotor 103rotates in direction B (which is the same as the direction of rotationof the main gear 150). Although FIG. 4 shows outer rotors 102 a, 102 brotating in a counter-clockwise direction and inner rotor 103 and maingear 150 rotating in a clockwise direction, it should be noted that theconverse is also possible (i.e. outer rotors 102 a, 102 b can rotateclockwise and inner rotor 103 and main gear 150 can rotatecounter-clockwise).

It should also be noted that in some embodiments, during operation ofmachine 100, the speed of rotation of each rotor 102 a, 102 b, and 103in each triplet 160 is the same magnitude. That is, rotors 102 a and 102b rotate at a given speed in a given direction of rotation, and innerrotor 103 rotates at substantially the same speed as outer rotors 102 a,102 b, but in the opposite direction. As noted above, the rotation ofrotors 102 a, 102 b, and 103 causes the rotation of outer gears 118 andinner gear 119, which in turn causes rotation of main gear 150. It isunderstood that the circumference of the main gear 150 at its outer face151 is greater than the circumference of the main gear at its inner face152. As such, the linear (tangential) speed of the outer face 151 willbe greater in magnitude than the linear (tangential) speed of the innerface 152 for a given angular velocity. Accordingly, it is understoodthat radially-outer gears 118, radially-inner gear 119 and faces 151,152 of main gear 150 may be sized and configured to accommodate thecommon rotation speed of radially-outer gears 118 and radially-innergear 119 that are meshed with common main gear 150. For example, thediameter of the inner gear 119 may be selected so as to be shorter thanthe diameter of the outer gears 118 a, 118 b in one rotor triplet 160.

It has been found that an increase in magnetic path utility due to such(e.g., triangular) triplet configurations can allow for significantsavings in weight and bulk, as compared to electric machines whichassign one magnetic circuit to each rotor. For example, the addition ofa third rotor (e.g. inner rotor 103) may cause an increase in poweroutput by up to 50% relative to a configuration including only rotorpairs sharing a magnetic circuit without the third rotor. As will beunderstood by those skilled in the relevant arts, an increase in thediameter of an individual rotor magnet 128, and the correspondingstrength of that magnet 128's surface area and the correspondingstrength of the magnet's electromagnetic interaction with itscorresponding winding 108 can be utilized to increase the power providedby machine 100. However, to optimize the power provided, thecross-section of the corresponding stator 122 may be increased, in orderto maintain the desired flux density. By grouping magnets 128 in rotortriplets 160 and employing a shared stator 122, flux density may bemaintained with a minimal weight penalty, which may be particularlyimportant in weight-critical applications such as aerospace andtransportation activities.

Windings 108 may be provided in any configuration suitable for use inaccomplishing the purposes described herein. A wide variety of suchconfigurations are known which may maximize the efficiency of machine100 for a given application. For example, single Litz or multiple strandwindings 108 may be used in configuring either machine 100, individualrotors 102, 103, rotor triplets 160, or other sets of rotors. The use ofmultiple windings 108 in machine 100 may be employed, for example inconjunction with a suitable mechanical indexing of the rotors 102, 103to fully or partially provide desired phasings in torque applied byrotors 102, 103 to main gear 150 and shaft 104. In some embodiments,three-phase windings may be employed.

As depicted in FIG. 5, in some embodiments the machine 100 includes onewinding 108 per stator 122. That is, one winding 108 is provided foreach triplet 160 of rotors 102 a, 102 b, 103. The use of a singlewinding 108 per rotor triplet 160 may provide improved efficiency forthe machine 100, as compared to machines in which multiple windings areused.

As noted previously, the efficiency of machine 100 can be increasedthrough the suitable phasing (i.e., indexing) of rotors 102, 103 withrespect to each other and with respect to winding 108. In particular,the operation of machine 100 can be controlled by phasing outer rotors102 a, 102 b and inner rotor 103 with respect to each other and towinding 108 in triplets. This may be accomplished, for example, bysuitable gearing of outer rotors 102 a, 102 b and inner rotor 103 withrespect to each other and to motor shaft 104.

In the example embodiment shown in FIGS. 3 and 4, each gear 118 drivenby outer rotors 102 engages the outer face 151 of main gear 150, andeach gear 119 driven by inner rotors 103 engages the inner face 152 ofmain gear 150, so that total torque applied to main gear 150 is the sumof the torques applied by the inner and outer gears 118, 119. Ifwindings 108 are configured substantially circumferentially about axis200 of shaft 104 and therefore machine 100, an index angle 112 may bedefined between equators 202 (a theoretical line dividing a magnet intoa north and south half) of individual magnets 128 and radii 204extending from axis 200 to a corresponding rotor 102, 103. By suitablearrangement of rotors 102, 103 and/or gears 118, 119, index angles 112may be set at desired values for individual rotors, and triplet setsthereof, with the result that phased torque output applied by each ofthe rotor triplets 160 can be applied to provide smooth, continuous, andpowerful torque to shaft 104 via main gear 150, in the case of motoroperation. In the case of generator operation, smooth and continuouscurrent output may be obtained from winding(s) 108 by applying a torqueto shaft 104.

FIG. 6 is a schematic front partial cut-away view of a portion of anelectric machine showing rotor orientations in accordance with anembodiment. Radius 204 a extends from axis 200 through the centre ofouter rotor 102 a. Radius 204 b extends through the centre of outerrotor 102 b. Radius 204 c extends through the centre of inner rotor 103.As depicted, equator 202 a of outer rotor 102 a is perpendicular toradius 204 a. Equator 202 b of outer rotor 102 b is perpendicular toradius 204 b, with the polarity of the magnet in outer rotor 102 b beingreversed relative to the magnet in outer rotor 102 a. Equator 202 c ofinner rotor 103 is parallel to radius 204 c. It will be appreciated by aperson skilled in the art that FIG. 6 is an example depiction of innerrotors 103 and outer rotors 102 a, 102 b at a moment in time, and thatthe rotors will be rotating throughout operation. The difference betweenthe index angles of each rotor 102 a, 102 b, 103 in a triplet may besubstantially maintained throughout operation, although there may bevariation by several degrees depending on the driving current, theloading on the machine 100, and the phase advance.

FIGS. 7A to 7E are schematic front partial cut-away views of the portionof the electric machine depicted in FIG. 6 throughout a half-cycle ofoperation (that is, throughout 180 degrees of rotation). FIG. 7Aillustrates the same initial configuration as FIG. 6, as well as arrowsindicating the outer rotors 102 a, 102 b moving in direction A, and theinner rotor moving in direction B, and the flux lines associated withthe magnetic circuit in stator 122.

FIG. 7B depicts the electric machine of FIG. 7A after 45 degrees ofrotation. As can be seen, each of outer rotors 102 a, 102 b has rotatedsubstantially 45 degrees in direction A (in this example, direction A iscounter-clockwise), and inner rotor has rotated substantially 45 degreesin direction B (in this example, direction B is clockwise). The fluxpath is also depicted.

FIG. 7C depicts the electric machine of FIG. 7A after 90 degrees ofrotation relative to the initial configuration in FIG. 7A. As can beseen, each of outer rotors 102 a, 102 b has rotated substantially 90degrees in direction A, and inner rotor 103 has rotated substantially 90degrees in direction B. It can be seen that at this moment in time, theflux path is temporarily broken between the three rotors 102 a, 102 b,103.

FIG. 7D depicts the electric machine of FIG. 7A after 135 degrees ofrotation relative to the initial configuration in FIG. 7A. As can beseen, each of outer rotors 102 a, 102 b has rotated substantially 135degrees in direction A, and inner rotor 102 has rotated substantially135 degrees in direction B. The flux path is also depicted and onceagain travels through each of the rotors in the triplet, although theflux now follows a clockwise path relative to the initialcounter-clockwise flux path in FIG. 7A.

FIG. 7E depicts the electric machine of FIG. 7A after 180 degrees ofrotation relative to the initial configuration of FIG. 7A. As can beseen, each of outer rotors 102 a, 102 b has rotated substantially 180degrees, and inner rotor 103 has rotated substantially 180 degrees. Assuch, each of rotors 102 a, 102 b and 103 is now in the oppositepolarity relative to the initial configuration of FIG. 7A, and the fluxpath depicted is substantially opposite in direction to the flux pathdepicted in FIG. 7A.

It will be appreciated that in FIGS. 4 and 5, the inner rotor angle isdifferent than the embodiments shown in FIGS. 6 and 7A-7E. It should beappreciated that the indexing of the inner rotor 103 relative to outerrotors 102 a, 102 b may be different than what is depicted in FIG. 7 byseveral degrees, as the configuration may vary depending on whether amachine is optimized for torque, speed, or the like.

In the embodiment shown in FIGS. 1 and 2, an 18-rotor (12 outer, 6inner), 6-phase system is shown. As will be understood by those skilledin the art, the disclosure is also applicable to a 9-rotor (6 outer, 3inner), 3-phase system, a 36-rotor (24 outer, 12 inner), 12-phasesystem, and other combinations.

In the case of the 18-rotor, 6-phase system depicted in FIG. 2, each ofthe 12 outer rotors and 6 inner rotors may be grouped into six triplets160, each triplet having 2 outer rotors 102 a, 102 b, and one innerrotor 103.

Further, each of the 6 rotor triplets 160 a may be phased relative toits adjacent two rotor triplets 160 b, 160 c. For example, equators 202of the 1^(st) and 4^(th) triplets 160 a, 160 d may be aligned with theirrespective radii 204 from axis 200 (though the outer rotors 102 a and102 b may be 180 degrees out of phase with one another), while equators202 of the 2^(nd) and 5^(th) triplets 160 b, 160 e are indexed by 60degrees with respect to the 1^(st) and 4^(th) triplets 160 a, 160 d, andequators 202 of 3^(rd) and 6^(th) triplets 160 c, 160 f may be indexedby 60 degrees with respect to 2^(nd) and 5^(th) triplets 160 b, 160 e,and by 120 degrees with respect to 1^(st) and 4^(th) triplets 160 a, 160d.

In a 9-rotor (6 outer, 3 inner), 3-phase system, each adjacent rotortriplet 160 may be indexed by 120 degrees with respect to its neighbourtriplets (this may be implemented as a single channel, 3 phase system).In a 36-rotor (24 outer, 12 inner) system, each adjacent rotor triplet160 can be indexed by 30 degrees relative to its neighbour triplets(this may be implemented as a dual channel 6-phase system, a 3-channel4-phase system, or a 4-channel 3-phase system). In an 18-rotor (12outer, 6 inner) system, each adjacent rotor triplet 160 can be indexedby 60 degrees relative to its neighbour triplets (this may beimplemented as a single channel, 6 phase system, or a dual channel,3-phase system).

As will be readily apparent to those skilled in the relevant arts, awide variety of combinations and geometries of indexing and phasing maybe chosen, depending on the desired input and output characteristics,and geometry, of the machine 100. For example, adjacent rotor triplets160 can be indexed relative to each other such that when a current ispassed through one or more windings magnetically coupled to therespective stators, the rotor triplets 160 provide phased rotary powerto the common main gear 150.

As will be further apparent to those skilled in the relevant arts, thedesired indexing of adjacent rotor triplets can be accomplishedmechanically, electrically, or in any suitable or desired combinationthereof.

As previously noted, in various embodiments this disclosure provideselectric machines having a plurality of flux paths (i.e., magneticcircuits) defined between triplets of rotors, each triplet of rotorsbeing associated with a shared stator 122. Respective triplets of rotorsmay further be associated with a single winding 108, shared by thetriplet.

Any materials suitable for use in accomplishing the purposes describedherein may be used in fabricating the various components of machine 100,including, for example, those used in fabricating analogous componentsof known electric machines. The selection of suitable materials would bewithin the knowledge of those skilled in the art.

As has already been noted, machine 100 may be operated as a motor byapplying a suitable AC or commutated DC voltage across windings 108, oras a generator by applying mechanical torque to shaft 104 and tappingcurrent from leads suitably connected to windings 108.

Electric machines in accordance with the disclosure can be operated,with appropriate rectifiers, solid state switches, capacitors, and otherelectronic components, using either direct- or alternating-currentinput, to provide either direct or alternating-current output, dependingupon whether electrical or mechanical input is applied to the windings108 or shaft 104, respectively.

The above descriptions are meant to be exemplary only, and one skilledin the art will recognize that changes may be made to the embodimentsdescribed herein without departing from the scope of the describedsubject matter. Still other modifications which fall within the scope ofthe described subject matter will be apparent to those skilled in theart, and such modifications are intended to fall within the scope of theappended claims.

Of course, the above described embodiments are intended to beillustrative only and in no way limiting. The described embodiments aresusceptible to many modifications of form, arrangement of parts, detailsand order of operation. The invention is intended to encompass all suchmodification within its scope, as defined by the claims.

What is claimed is:
 1. A multi-rotor electric machine comprising: astator; a first rotor magnetically coupled to the stator and rotatablymounted relative to the stator; a second rotor magnetically coupled tothe stator and rotatably mounted relative to the stator; and a geardrivingly coupled to both the first and second rotors, the first rotorbeing drivingly coupled to a first face of the gear and the second rotorbeing drivingly coupled to a second face of the gear.
 2. The electricmachine of claim 1, wherein the first face of the gear is aradially-outer face, and the second face of the gear is a radially-innerface.
 3. The electric machine of claim 1, further comprising a thirdrotor magnetically coupled to the stator and rotatably mounted relativeto the stator, the third rotor being drivingly coupled to the gear. 4.The electric machine of claim 3, wherein the first, second and thirdrotors are magnetically indexed in a triplet sharing a common magneticcircuit.
 5. The electric machine of claim 3, wherein the third rotor isdrivingly coupled to the first face of the gear.
 6. The electric machineof claim 5, comprising a plurality of triplets of first, second andthird rotors, the first face of the gear is a radially-outer face, andthe second face of the gear is a radially-inner face.
 7. The electricmachine of claim 1, wherein the first and second rotors are configuredto rotate in opposite directions.
 8. An electric machine comprising: aplurality of stators circumferentially distributed about a central axisof the electric machine; a plurality of rotors arranged in triplets,each triplet of rotors sharing a common magnetic circuit with arespective one of the stators, each triplet of rotors comprising anadjacent pair of radially-outer rotors and a radially-inner rotorrelative to the central axis; and a common gear rotatable about thecentral axis and drivingly coupled to the plurality of magnetic rotors,the radially-outer rotors being drivingly coupled to a radially-outerface of the common gear and the radially-inner rotors being drivinglycoupled to a radially-inner face of the common gear.
 9. The electricmachine of claim 8, wherein each triplet of rotors is configured to beseparately controlled via the respective shared common magnetic circuit.10. The electric machine of claim 8, wherein adjacent triplets of rotorsare indexed relative to each other such that when a current is passedthrough one or more windings magnetically coupled to the respectivestators, the triplets of rotors of that stator provide phased rotarypower to the common gear.
 11. The electric machine of claim 8, whereinthe radially-outer rotors and the radially-inner rotor of one or more ofthe triplets of rotors are configured to rotate at substantially a samerotational speed when being drivingly coupled to the common gear. 12.The electric machine of claim 8, wherein the radially-outer rotors andthe radially-inner rotor of one or more of the triplets of rotors areconfigured to rotate in opposite directions when being drivingly coupledto the common gear.
 13. The electric machine of claim 12, wherein theradially-outer rotors and the radially-inner rotor of one or more of thetriplets of rotors are configured to rotate at substantially a samerotational speed when being drivingly coupled to the common gear. 14.The electric machine of claim 8, wherein each triplet of rotors isenergized by a single winding.
 15. A method of operating an electricmachine having a stator, a first rotor magnetically coupled to thestator and a second rotor magnetically coupled to the stator, the methodcomprising: driving a gear via a first face of the gear using the firstrotor; and while the gear is driven using the first rotor, driving thegear via a second face of the gear using the second rotor.
 16. Themethod of claim 15, wherein the first face is a radially-outer face ofthe gear and the second face is a radially-inner face of the gear. 17.The method of claim 15, wherein the first rotor and the second rotorrotate in opposite directions.
 18. The method of claim 15, wherein thefirst rotor and the second rotor operate at substantially a samerotational speed.
 19. The method of claim 16, comprising driving thegear via the radially-outer face of the gear using a third rotor whilethe gear is driven using the first and second rotors, the third rotorbeing magnetically coupled to the stator.
 20. The method of claim 19,wherein: the first and third rotors rotate in a first direction; and thesecond rotor rotates in a second direction opposite the first direction.